I. Field of the Invention
This invention relates generally to the sterilization of items using a gas or vapor as a sterilant and more particularly to determining the concentration of one or more gases in a sterilization chamber.
II. Description of the Prior Art
The surfaces of virtually all objects are covered with transmissible agents such as fungi, bacteria and viruses. It is often necessary to sterilize objects such as food products, packaging, biological materials and medical implements to eliminate or make sterile such transmissible agents. Various methods for sterilizing objects have been used in the past. Known methods of sterilization include heating and chemical treatments.
Heat sterilization involves applying steam or dry heat to the objects to be sterilized for a suitable period of time. While this method of sterilization is effective for many objects, heat sterilization is not suitable for objects adversely affected by heat. Objects subjected to heat sterilization can reach 100° to 120° C., temperatures sufficiently high to cause damage to the object. Further, heat sterilization often requires large amounts of electrical power and water. These resources are not always readily available in remote locations such as a military field setting. Chemicals which have been used in the past to sterilize objects include alcohols, aldehydes, phenols, ozone, ethylene oxide, and hydrogen peroxide. Sterilization using chemicals can be accomplished at lower temperatures and can be highly effective when sterilizing heat-sensitive items. However, care must be taken to ensure all surfaces are sterilized. This is a difficult task when sterilizing catheters, tubing, and other objects with small, difficult to reach spaces. To penetrate into such spaces, chemicals in a gaseous or vaporous form have been used as sterilants.
Various gases and vapors have been used as a sterilant when sterilizing heat sensitive objects. (The words “gas” and “vapor” in their singular and plural form will be used interchangeably hereinafter to refer generically to both gases and vapors). Proper care and handling of such sterilants is crucial because of their potentially toxic nature. Using hydrogen peroxide gas as a sterilant offers certain advantages. First, it has non-toxic properties at low concentrations and therefore is not prohibitive to human handling. Second, at low concentrations hydrogen peroxide is non-corrosive and can therefore be stored for long periods of time. Even at higher concentrations suitable packaging can be employed to protect humans from exposure. When properly packaged, the shelf-life of hydrogen peroxide sterilant solutions can be multiple years in length. Third, hydrogen peroxide degrades into water and oxygen, two non-toxic byproducts. Fourth, sterilization using hydrogen peroxide gas as a sterilant can be performed at lower temperatures (55° to 60° C.) than heat sterilization. Virtually all products requiring sterilization are not adversely affected by temperatures in this range. Fifth, hydrogen peroxide gas sterilization requires less energy and essentially no water than heat sterilization.
When hydrogen peroxide gas is used as a sterilant, the concentration of hydrogen peroxide gas in a sterilization chamber must be maintained within a specific range for a predetermined time to ensure proper sterilization. The most efficient and effective hydrogen peroxide concentration range and time are dependent on the materials to be sterilized, the sterilization load and other environmental and operational factors. For this reason it is important to accurately monitor the hydrogen peroxide concentration throughout the sterilization process. The same is true for other gas sterilants.
A variety of techniques have been employed in the prior art to determine the concentration of hydrogen peroxide in a sterilization chamber. Various chemical techniques for measuring concentrations of hydrogen peroxide vapor or gas are disclosed in U.S. Pat. Nos. 6,491,881 and 6,953,549. Electrical techniques for measuring concentrations of hydrogen peroxide vapor or gas are disclosed in U.S. Pat. Nos. 6,933,733 and 6,946,852. Thermal techniques for measuring concentrations of hydrogen peroxide vapor or gas are disclosed in U.S. Pat. No. 4,843,867. All of these techniques require sophisticated calibration methods and suffer from their invasive nature.
Optical techniques have also been employed to determine the concentration of hydrogen peroxide gas and water vapor within a sterilization chamber. When such optical techniques are used, an attempt is made to measure absorption of electromagnetic radiation by the gaseous contents of the sterilization chamber and calculate concentration based upon the measured amount of absorption. Hydrogen peroxide gas is known to strongly absorb light in portions of the ultraviolet spectrum. Water vapor is known to strongly absorb light in portions of the infrared spectrum. However, these prior art techniques fail to fully account for factors that can affect the measurements taken and, thus, any calculation of concentration.
U.S. Pat. Nos. 5,600,142; 5,847,392 and 5,847,393 disclose methods which employ a spectrometer to take measurements of various wavelengths of the spectrum (e.g. 1420 nm, 915-959 nm, 1350-1400 nm, and/or 1830-2000 nm). From these readings, an effort is made to determine the concentration of water and hydrogen peroxide in the chamber. The methods disclosed in the patents are very expensive to implement with slow response times. Also, the methods described fail to account for factors other than concentration that can affect the measurements and, thus, the calculation of concentration.
U.S. Pat. No. 6,875,399 discloses a sterilization system including a sensor for detection of a component, such as hydrogen peroxide vapor, in a multi-component vapor, such as a mixture of vapor hydrogen peroxide and water supplied to a chamber of the system. The sensor preferably uses a wavelength range in which hydrogen peroxide strongly absorbs, but other components of the vapor, such as water, do not. While this avoids the need to use complex subtraction procedures normally used to remove the contribution of water or other substances from detected absorbance measurements, it fails to address other factors that can affect the measurement. For example, U.S. Pat. No. 6,875,399 discloses a probe within the sterilization chamber that can become coated with materials that affect its performance. Also, the system disclosed in the patent does not account for other factors that can affect the measurements such as changes in the output of the light source, light from other sources or other factors that can affect light absorption measurements.
U.S. Pat. No. 7,157,045 discloses a vaporizer that supplies hydrogen peroxide and water vapor to a high level disinfection or sterilization region. Light detectors detect light which has traversed a region of the treatment chamber in a first narrow portion of the spectrum which is absorbed by the hydrogen peroxide vapor, a second narrow portion of the spectrum which is absorbed by the water vapor, and a third narrow portion of the spectrum that is absorbed by neither the hydrogen peroxide vapor nor the water vapor. From these measurements, an absorbance or transmittance is measured from which the concentrations of hydrogen peroxide and water vapor are determined. The detector used to detect light in the third portion of the spectrum is intended to provide background intensity readings. Again, this system suffers from problems discussed above. Also, the light in the third portion of the spectrum can be absorbed by the parts or contents of the chamber to some degree resulting in incorrect calculations of concentration. Diagnosing problems with the chamber, its contents and the concentration measurement system itself is made more difficult by the design of the measurement system of U.S. Pat. No. 7,157,045.
U.S. Pat. No. 6,269,680 discloses an attempt to use ultraviolet light to determine the concentration of hydrogen peroxide vapor. However, the lamps disclosed for use in this patent vary in their output as they warm-up and therefore introduce the potential for creating luminosity variations that affect the measurement. Further, the lamps disclosed for use emit large amounts of heat creating a need to maintain a constant temperature in the measurement device to obtain an accurate measurement. This requirement to maintain temperature adds to the complexity and cost of the system.
In view of the foregoing, there is a real need for a non-invasive, real-time, low-cost and accurate method and apparatus for determining the concentration of a gas sterilant in a sterilization chamber. Likewise, there is a real need for such an apparatus and method that can also measure the concentration of water vapor or other gasses in the sterilization chamber. Further, there is a need for a concentration measurement system that allows for accurate evaluation and diagnosis of problems that may arise during sterilization. These needs are fully addressed by the present invention.